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“drought tolerant” = misleading bull

I’ve had a few conversations recently that revealed to me what a dumb thing it is to call a plant “drought tolerant”. Wikipedia gets this right technically, but leaves us without enough to apply the information: “Drought tolerance refers to the degree to which a plant is adapted to arid or drought conditions.”

Well, okay – so when we say a plant is drought tolerant, what the heck does that mean? It means that plant has some degree of adaptability to drought. But here’s the thing – how much adaptability? How do I use this information? What does this plant do in response?

Iris douglasiana

Saying a plant is “drought tolerant” is like saying there is a temperature outside. Is it cold, warm or hot? See what I am getting at? “drought tolerant” only means that there is some degree to which the plant may tolerate (not die immediately) a shortage of water.

More information is needed: enter WUCOLS. I won’t bore you with the details and technical stuff behind WUCOLS, they do well enough – read through their website. Not bored yet? Good! Now read through the rules on WELO, the California ordinance for water efficiency in the landscape. Bet you’re bored now.

The deal is that these classifications in WUCOLS mean something. They quantify how much water a plant wants in a particular region of California. This does not take into account sun/shade exposure, sun orientation, wind, soil type, or any of the other factors involved in meeting a plant’s water needs. It is not a perfect tool, but it is the best thing we have going. WUCOLS isn’t the only resource, you can get information from nurseries and books also, but it is the most helpful hydrozoning tool I use.

In WUCOLS, the classifications are “very low”, “low”, “moderate”, “high”, or it may say “inappropriate” or “unknown”. This is pretty self-explanatory, but what is most interesting here is that ETo is a reference point – a baseline of sorts. So a “high” water use classification means 70-90% of the water needed to keep a 4″ tall cool season turf VERY well watered and super lush, or to put it another way, the “high” water classification is 70-90% of a whole lot of water.

I indulged online – I pulled up an article on “drought tolerant” plants from Sunset magazine. Of their 12 plants in my home town of Oakland, one was classified by WUCOLS as “very low”, one was “moderate”, and the rest were “low”. Nassella tenuissima / Mexican Feathergrass is listed as an invasive plant by PlantRight and should not be used at all, but WUCOLS isn’t about invasiveness, it is about water, so shame on Sunset!

It is bad juju to mix water use classifications. If you absolutely must, then the classifications should be next door neighbors – so “low” and “moderate” is kinda okay (provided you irrigate at the higher amount), but “moderate” and “very low” is no bueno.

Next I went to the “drought tolerant” poster child: succulents. Low water use, right? Drought tolerant?! erm…. no. Again, WUCOLS demonstrates my point. In the plants WUCOLS lists as succulents, the same 3 classifications are included – very low, low, and moderate (screen shot is just of the alphabetical early birds, I didn’t pick favorites here). So much for that assumption.

So really, drought tolerant is anything that will grow with less than 70% of a boatload of water. That could still an incredible amount of water, folks. Granted, the lower classifications are pretty good and some plants truly do have a relatively dry footprint, but all plants need extra water during their establishment period, that is a post for another day.

Oh – and about those drought tolerance adaptations emphasized in the first paragraph: a drought tolerant plant might go dormant, drop all or some of its leaves, wilt, or shrink in size in times of stress (the drought they’re tolerating) as a way of coping …. and if not watered in time, drought tolerant plants die. Tolerance is about not being dead yet – it has nothing to do with living well, being healthy, or looking good, it is about living to drink another day.

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4 Responses

Excellent article. I’ll add some frustration with the word “drought.” It seems to imply that a drought is an unusual event, and It Will Be Over One Day. Whereas in so much of CA, we are living beyond our means as regards water. Sure, we have a drought — but that’s on top of our already low and also seasonal water climate. I think we should be using Low and Very Low water use plants as a matter of course, not waiting for the drought to be over, but because they suit the climate we’re living in. Which to folks in other climates looks like a drought all the time….

Tina – Agreed! At the moment with the new MWELO, we’re still allowed to have moderate and even high water use plants provided we meet the budget, but that doesn’t make it the best, only where we are legally right now.
Jen

Agreed that we need a different name, summer dry? climate appropriate? I was wondering about the inclusion of the Euphorbia hybrids as succulents. While some Euphorbias are succulent and need little to no water, the hybrids need a fair amount of watering to make it through the dry season.

Robin – You are so right. What I think we need is to stop relying on buzz words and be patient with sifting through specifics. I’ll be posting very soon about succulents (by way of a tirade against common names), you may get a kick out of it when I put it up.
J